1. Field of the Invention
This invention relates in general to well production systems for subterranean resources such as oil or water, and in particular to equipment and methods for protection of metallic well casing from the corrosive effects of moist soil the well cellar area.
2. Description of the Related Art
Steel components of well production systems, such as the well casing, that are submerged in a corrosive environment require some form of protection to prevent corrosion. Cathodic protection (“CP”) systems, for example, are conventionally used to protect steel components of well production systems from corrosion.
One particular type of CP system is known as a galvanic anode cathodic protection (“GACP”) system. In GACP systems, steel structures can be protected from corrosion (“a protected metal”) by being positioned as a cathode in an electrochemical cell that includes an anode composed of a more highly reactive metal than the cathode. The anodes can be composed, for example, of highly reactive metals such as aluminum, zinc, or magnesium. The electrochemical cell includes an electrolyte (e.g., water or moist soil), and the anode and the cathode are positioned in the same electrolyte to provide an ion pathway between the anode and the cathode. In the electrochemical cell, the anode and the cathode are also eclectically connected, for example, by a conductive cable, to provide an electron pathway between the anode and the cathode.
When the protected metal and the anode are positioned in the electrochemical cell accordingly, the more reactive anode corrodes in preference to the protected metal structure, thereby preventing corrosion of the protected metal. Due to the difference in the natural potentials between the anode and the protected metal, by their relative positions in the electro-chemical cell, when the anode corrodes, high-energy electrons flow from the anode to the cathode through the electrical connection, thereby preventing an oxidation reaction at the protected metal structure. Thus, the anode corrodes instead of the protected metal (the cathode), until the anode material is depleted. The anode in a GACP system is known as a “sacrificial anode,” and likewise, GACP systems are also known as “sacrificial anode systems”
GACP systems are conventionally used for the cathodic protection of subsea pipeline due to the high conductivity of seawater and the ease at which galvanic anodes can be placed on the pipeline. On the other hand, GACP systems are not primarily used for cathodic protection of subterranean well casings because of the higher current output necessary to protect large metal structures surrounded by a highly resistive ground electrolyte.
Another type of CP system is known as a impressed-current cathodic protection (“ICCP”) system. In many ways, ICCP systems are similar to GACP systems —except that ICCP systems use less reactive anode metals needing to be connected to an external power source to provide greater current output. In the prior art, ICCP systems have been used for the purpose of protecting subterranean well casings in well production systems. Impressed-current cathodic protection systems employ D/C power (e.g., rectified A/C power) to impress a current between one or more external anodes (e.g., positioned in a subterranean anode bed) and the cathode surface (e.g., a well casing). The anode bed and the well casing are both buried in the earth, and are surrounded by a ground electrolyte (e.g., backfill for the anode bed or moist soil for the well casing).
Although ICCP systems are intended to protect the entire length of subterranean pipeline in typical well production systems, ICCP systems often fail, however, to adequately protect certain sections of the well casing, such as those sections that are shielded from the ground electrolyte. In particular, certain sections of the well casing, for example, those sections enclosed by a cellar (such as a circular metallic or non-metallic ring, such as a cement ring, installed at the wellhead base prior to drilling operations to secure the hole during drilling and are left in place during well production operations) at or near the earth surface, for example, are shielded from the ground electrolyte, and thus, are inadequately protected by conventional ICCP systems. Those certain sections of the well casings enclosed by a cellar can be, for example, the upper two meters of the well casing.
One conventional use of GACP for cathodic protection of subterranean well casings has been to provide a supplemental cathodic protection system to a relatively small number of well casing joints or sections at or near the surface of the well casings, leaving the remainder of the well casing to be protected by other means, such as an ICCP system. Accordingly, combined ICCP-and-GACP systems have been used to provide overall protection of the well casing as well as localized protection of the well casing sections in the cellar area. Conventional GACP systems used for this purpose have included standard cylindrical anodes, for example, two pre-packaged 60-lbs. magnesium anodes, positioned within the cellar. Because the relatively low current output of GACP systems compared to ICCP systems, and because of the relatively high level of non-homogenous electrolyte in the cellar area, there are unique disadvantages of using GACP systems in the cellar area. The electrolyte in the cellar area, which is also referred to as the “backfill,” can become polluted, for example, with various non-conductive or less conductive substances (herein referred to as “non-homogenous”) such as drilling mud, cement, or other foreign particles. A non-homogenous backfill disadvantageously increases the resistance within the electrochemical cell and reduces the effectiveness of the GACP system. A related disadvantage, is that due to increasing non-homogeneity of the backfill, the ongoing effectiveness of the GACP system is reduced over time and, eventually, the backfill must be replaced periodically to restore an adequate level of cathodic protection, which can be both time-consuming and costly, and failure to periodically replace the backfill has resulted in significant corrosion to metal structures in the cellar area, resulting in even more time-consuming and costly repairs of the upper well casing joints. There is a need in the art for improved cathodic protection systems for well casing sections in the cellar area that exhibit greater effectiveness in polluted backfill and reduce or eliminate the need to periodically replace the backfill.